An operating Light Emitting Diode (LED) lighting device requires an energy efficient and cost effective electronic driver circuit. Currently, the most common solution is to apply pulse width modulation (PWM) to LED lighting devices to generate constant forward current and to control brightness by adjusting PWM duty cycle with a number of available options such as, among others, buck, buck-boost, flyback, and SEPIC (Single Ended Primary Inductor Converter) and other topologies.
While the PWM principle has been well known for decades, an off line LED driver (driver that receive power from a household power line which ranges from 100 volts to 230 volts rms) application presents a particular challenge due to high switching voltages (e.g., 170V-330V peak-to-peak depending on line voltage and driver topology). Regardless of topology, an LED driver utilizes a power switch such as a power FET (Field Emission Transistor) to control the driving current provided to the LED lighting devices. Switching losses for such power FET are proportional to PWM frequency parasitic capacitance and a square of voltage. In addition to FET parasitic capacitance, diode reverse recovery losses increase with high frequency as well. Since a higher switching voltage increases the losses dramatically, a low PWM frequency is typically used with conventional switching components. A typical off line LED driver uses PWM frequency in a range between 50 KHz and 150 KHz.
However, using such a low PWM frequency requires high value passive components, such as capacitors and inductors. High value passive components present a number of drawbacks such as high cost, large size, and high parasitics. In some applications high value/high voltage capacitors can be only of electrolytic type to meet cost requirements. However, electrolytic capacitors are particularly unsuitable for off line retrofit LED lighting because of reduced lifetime at high temperature, which is far shorter than the expected lifetime of 50,000 hours for an LED source.
Thus, it would be desirable to provide a high voltage LED driver circuit which provides highly efficient switching and yet has low cost, reduced size, low parasitic resistance and high reliability. It would be also desirable to provide such an LED driver circuit in a compact advanced packaging to further improve LED driver efficiency.